The invention relates to a directional control valve which has a movable control piston which separates two pressure chambers from each other in the valve housing interior, which pressure chambers can be subjected to control pressure in order to move the control piston into axial positions corresponding to relevant switch positions of the valve in accordance with the difference in pressure prevailing between the control chambers.
Directional control valves of this type are known. For example, DE 197 10 318 A1 shows a valve of this type in the form of a three-way proportional fitted valve having an inductive travel sensor for detecting the position of the control piston and an added pilot valve via which the pressure chambers for controlling the position of the control piston can be subjected to control pressure in order to move the control piston into the relevant axial positions which correspond to the desired switch positions of the valve.
In the case of valves of this type, if, for some reason, no control pressure acts on the control piston, whether because the system which the valve forms part of is switched off and there is no system pressure, or whether a power failure at the pilot valve or emergency tripping causes the hydraulic short-circuiting of the pressure chambers at the control piston, then the pressure piston takes up an undefined, axial position. When the system pressure is switched on, a consumer which is connected may therefore execute an undesired, uncontrolled movement, which may constitute a risk of an accident or may result in a breakdown.
The invention is based on the object of providing a directional control valve in which the above-mentioned problems resulting because of a cessation of the control pressure are avoided.
In the case of a directional control valve of the type mentioned at the beginning, this object is achieved according to the invention by the fact that an arrangement is provided which acts on the control piston, produces an actuating force and prestresses said control piston for movement into an axial position corresponding to a desired predetermined position.
This ensures that even when there is no difference in pressure between the pressure chambers which can be subjected to the control pressure, the control piston takes up a position at which the system pressure can be switched on without any risk and without particular safety measures having to be taken.
When there is no difference in pressure in the pressure chambers, the desired position to be taken up by the control piston preferably corresponds to a position at which the valve forms a connection between a consumer connection and a tank connection. In this arrangement, if there is a cessation of the control pressure, a consumer which is connected comes into its final operating position, i.e. it moves to its end stop.
So that this process takes place without any risk, i.e. the consumer only moves slowly to its end stop, in a particularly advantageous exemplary embodiment the arrangement is made in such a manner that the arrangement producing the actuating force exerts a force on the control piston that counteracts flow forces which act on said control piston when pressure medium flows from the consumer connection to the tank connection, and that the size of the actuating force is selected to be just sufficient so that the flow forces, from a certain strength of the pressure-medium flow, compensate for the action of the actuating force on the control piston. The effect achieved by this is that when there is a lack of difference in the control pressure, the control piston is opened by the actuating force, which acts on it, with the effect of opening the connection from the consumer connection to the tank connection only to an extent sufficient for a certain strength of the pressure-medium flow to the tank connection to arise, at which strength the flow forces acting here on the control piston compensate for the actuating force. As a result, a state of equilibrium arises at a desired pressure-medium flow, which can be selected by the strength of the actuating force, from the consumer to the tank. With appropriate selection of the strength of the actuating force, which prestresses the control piston, this state of equilibrium corresponds to a strength of the pressure-medium flow at which the relevant consumer moves only very slowly to its end stop.
A spring arrangement can be provided to produce the actuating force acting on the control piston. In an advantageous exemplary embodiment in which the control piston forms, at its front end, a pipe valve which is guided in a valve bushing and whose open, front end is connected to the consumer connection and which has a control aperture which produces the connection to the tank connection in an axial position slid forward out of the valve housing, the spring arrangement is formed as a compression spring which is supported at one end on the tear end of the control piston and at the other end on the inner end of the valve housing interior. The spring force is selected in such a manner that even at a low strength of the flow of the pressure medium flowing into the open, front end of the pipe valve, the flow forces which are effective on the control piston equalize the spring force, so that the above-mentioned state of equilibrium arises even at a small pressure-medium flow.